Initial attempt to interests implementation

-- Provided a new EurekaRegistry API as I did not want a lot of conflicts for David. This can merge into existing registry implementation later.
-- Provided an Index and interest abstraction.
-- Had to comment a test case for EurekaTransport because avro was not able to handle an abstract class (why?)
-- Created a new module eureka-test-utils that will be used to provide test utilities for eureka for internal (eureka project) and external (people using eureka) use.